Alloys containing nominally 6% aluminium, 5% zirconium, about 0.2% silicon, with or without carbon, and up to about 2% of tungsten have been proposed for use in aircraft engines where service temperatures of up to 520.degree. C are encountered. Dimensional stability in such conditions is of the greatest importance and tolerances are extremely small. The alloys above-mentioned possess excellent high temperature strength but experience has shown that they are not always sufficiently resistant to embrittlement on exposure to high temperatures. Embrittlement in this context means loss of ductility measured at room temperature before and after exposure to high temperatures.
In the fields of application for which these alloys are intended, the relatively high strength must be combined with room temperature ductility of at least 15% reduction in area and 10% elongation measured on a gauge length of 4.sqroot.S.sub.o, where S.sub.o = cross-sectional area. Furthermore, these values must be retained after exposure to service temperatures since in practice alloys used in aircraft engine components are subjected to thermal cycles between normal atmospheric temperature and service temperatures of at least 500.degree. C. At the same time, creep resistance of a very high order is required and, since shaping is involved, forgeability must be good to avoid any possibility of cracking. Welding is frequently used in fabrication of parts made from such alloys and there must be no embrittlement as a result of local heating to welding temperatures and a high degree of weldability is required. Titanium alloys for the advanced aircraft engines now required have to meet a criterion in creep properties of 0.1% strain in 100 hours exposure to a stress of 20 tonf/in.sup.2 at a temperature of 520.degree. C.
Titanium alloys which are intended for use at elevated temperatures have to have a range of properties very different from those alloys which are only for use at room temperature. Such room temperature alloys are exemplified by, for example, the constructional alloy disclosed in British Patent Specification No. 1,079,416. This discloses a range of compositions, but the preferred composition is a 6% aluminium, 5% zirconium, 4% molybdenum, 0.2% silicon, 0.1% carbon. This alloy is given a heat-treatment which comprises heating the alloy to 900.degree. C, water quenching and aging for 24 hours at 500.degree. C. The result of the heat-treatment is that the alloy is utilised in the alpha plus beta condition, and the alloy obtains its strength from the hardening of the beta phase by the molybdenum. As can be seen from Table I in the British Patent, the ductility of the 4% molybdenum alloy is reasonable provided the percentage of carbon present is kept to a reasonable level. Thus in the alpha plus beta condition in which the alloy of the British Patent works, there is no need to reduce the molybdenum content to obtain good ductility. Such constructional alloys are concerned with properties such as forgeability and high tensile strength rather than with high temperature strength, and Table II of the British Patent is an example of the emphasis placed on forgeability in such an alloy.
The particular family of properties required by a titanium alloy for use in high temperature high stress conditions include weldability, high creep strength, good post-creep ductility, and good tensile and proof stress properties amongst many others. However, two extremely important properties, high creep strength and good post-creep ductility are not required by constructional alloys.
Titanium alloys used for high temperature applications may suffer from many defects, and in addition to the requirements for weldability, high creep strength, good post-creep ductility and good tensile and proof stress properties, a resistance to ordering is required. As aluminium and tin have an analagous effect on ordering, and 3% tin is equivalent to 1% aluminium, this equivalence has become an accepted factor in considering titanium alloys generally. In the past tin and aluminium have been considered mutually replaceable at will. Thus in French Patent No. 1,477,221 which is concerned with a creep resistant high temperature alloy, the specification refers to the use of both tin and aluminium, and it may have been thought that these two elements could be interchanged at will. It has now been discovered that this is not always so.
The present invention is concerned with an alloy having good stability to prolonged exposure to elevated temperatures under stress.